This invention relates to methods and apparatus for reducing the distortion appearing in an output signal which a signal handling means (such as an amplifier) produces in response to an input signal. In particular, this invention relates to predistorting the phase of an input signal to linearise the output of a signal handling means.
Amplifier predistorters are known in which a predistortion signal is combined with the amplifier input to predistort it in such a way as to counteract distortion imposed upon the input signal by the amplifier itself. Such predistorters may be complex and unsuited to integration.
It is an object of the invention to provide a relatively simple distortion reduction mechanism, such as a predistorter.
According to one aspect, the invention provides distortion reduction apparatus for reducing the distortion appearing in an output signal which a signal handling means produces in response to an input signal, wherein the input signal is provided to the apparatus in a digital format and the apparatus comprises data storage means for storing phase shift data and outputting phase shift data when addressed by the digital magnitude of the input signal and phase shift means for phase shifting the input signal under the control of the phase shift data to produce a predistorted input signal, wherein the phase shift means operates on the input signal in the analogue domain.
According to another aspect, the invention provides a method of reducing the distortion appearing in an output signal which a signal handling means produces in response to an input signal, said input signal being provided in digital format and the method comprising addressing a data storage means using the digital magnitude of input signal such that the data storage means outputs corresponding phase shift data and using the phase shift data to control a phase shift applied to the input signal to produce a predistorted input signal to counteract said distortion, wherein the phase shifting step comprises phase shifting the input signal in the analogue domain.
The distortion reduction scheme according to the invention is advantageously relatively simple. It is flexible because the phase shift data may be re-writable, for example, to cope with changing conditions or for calibration purposes. The invention uses the storage means to apply phase predistortion to the input signal to counteract the phase distortion of the input signal that occurs within the signal handling means. This may lead to a significant beneficial reduction in the AM-PM distortion and reduced spectral re-growth and error vector magnitude (EVM) in the output signal.
Some examples of applications where this approach may be particularly useful, include the linearisation of amplifiers and devices where AM-PM distortion is the dominant distortion mechanism and for certain modulations, where the control of phase distortion is particularly important.
Digital magnitude values of the input signal are used to address the storage means, resulting in an implementation particularly suited to integration.
Certain embodiments of the invention operate on analogue input signals. Such embodiments use an amplitude detector to measure the instantaneous amplitude of the input signal, this amplitude signal being used to address the storage means.
In a preferred embodiment, the storage means is a lookup table, which stores data corresponding to desired phase shift means control signals, the data being addressed by the corresponding input signal amplitude values. In general, lookup tables can be rewritten to adapt the predistortion to new operating parameters.